Treating for Mites

Last week I noticed some mites on one of my chickens. I ordered Elector PSP, it arrived today, and tomorrow I'm going to tackle spraying all of my chickens and coop. Before I begin though I have a few questions... Do just spray the neck and the vent area on the chickens, or do I need to get their entire body? I'm planning on cleaning out the entire coop, and nesting boxes and spraying them down. About 2 weeks ago we added sand for the first time underneath our roosts and in their outdoor area. Do I need to remove the sand from the coop to spray underneath it, or can I just spray on the sand? Do I need to spray outside too? Also, is it likely the mites came in the sand, or is it just a coincidence? I've been checking the chickens every month or so, and it seemed like the mites showed up right after the sand. Thanks!